Vehicle safety airbags have been generally available since the 1970s, and automakers have improved their cost and efficacy over time. Airbags can be equipped in multiple compartments of a vehicle, though the most common placement are the steering wheel hub and front passenger instrument panel. While the idea of an airbag in a car may seem simple, it takes several components, made of several different materials, for the safety mechanism to function.
Sensors measure the severity of the crash to determine if the impact is enough to require airbag inflation. The sensor is a small ball made of steel that is held in place by a spring or a magnet. The ball stays in place unless the vehicle decelerates very rapidly. If the impact is above the deployment threshold, the airbag will inflate. The threshold is between eight and 16 miles per hour, depending if the person in the vehicle is wearing a seat belt, with the higher threshold for belted occupants. When the sensor is activated and the steel ball moves, this turns on an electrical circuit.
The bag itself is generally made of a synthetic fabric such as polyamide or nylon. There are small vents within the weave of the fabric, allowing the gas to escape slowly so that it acts more as a soft cushion, rather than a hard barrier. It only takes a few seconds for the pressure inside the bag to decrease to that of atmospheric pressure.
Hybrid inflators use neutral chemicals like argon (Ar) in a canister, which is heated and released by a pyrotechnic charge. These are typically used in smaller airbags, like passenger-side airbags. Argon is a neutral gas, which does not combust or react, and is typically found in incandescent lights or wherever a completely neutral environment is needed.
A small pellet of sodium azide(NaN3) is ignited after the steel sensor activates the electrical circuit. The ignition of this chemical causes a chemical reaction, resulting in nitrogen gas. It is this nitrogen gas that inflates the airbag itself. This gas is harmless, and fills the bag in less than a second. Generally, the entire process from sensor activation to bag deployment takes approximately one-twentieth of a second.
However, it is a highly toxic chemical, with such products as sodium hydroxide (NaOH) and ammonia. Drivers frequently complained of burning in the lungs, nostrils, mouth, and skin.Sodium azide(CAS number: 26628-22-8) is being replaced over time by less dangerous compounds, like guanidine nitrate.
The simplest airbag, called a blow-down or blown-down system, uses compressed helium without a pyrotechnic chemical. A bottle ruptures and rapidly inflates the bag. These are found chiefly in rollover curtain systems, that is, in the head space of a sport vehicle, to prevent injury in case of a rollover.